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A that has a two-part hinged shell.

Then, even if circumstances change such that it no longer provides any survival or reproductive advantage, the behavior will still tend to be exhibited -- unless it becomes positively disadvantageous in the new environment.

The diversification, over evolutionary time, of a or group of species into several different species or subspecies that are typically adapted to different ecological (for example, Darwin's finches).

A recent book, , offers an introduction to the subject of memes.

In 1991, Kimbel and Yoel Rak found a 70 percent complete skeleton of .

The second highest level of taxonomic classification of organisms (below domains).

Compare with . A scientist who studies plants.

Among her research interests is a study of Nicaraguan Sign Language.

An anatomist, Kimbel worked with Don Johanson and assembled Lucy's skull fragments.

It cannot be used to determine the age of Earth, for example.

CORRECTION: Genetic drift has a larger effect on small populations, but the process occurs in all populations  large or small. Genetic drift occurs because, due to chance, the individuals that reproduce may not exactly represent the genetic makeup of the whole population. For example, in one generation of a population of captive mice, brown-furred individuals may reproduce more than white-furred individuals, causing the gene version that codes for brown fur to increase in the population  not because it improves survival, just because of chance. The same process occurs in large populations: some individuals may get lucky and leave many copies of their genes in the next generation, while others may be unlucky and leave few copies. This causes the frequencies of different gene versions to "drift" from generation to generation. However, in large populations, the changes in gene frequency from generation to generation tend to be small, while in smaller populations, those shifts may be much larger. Whether its impact is large or small, genetic drift occurs the time, in populations. It's also important to keep in mind that genetic drift may act at the same time as other mechanisms of evolution, like natural selection and migration. To , visit Evolution 101. To , visit this advanced article.

An early experimental , his 1901 paper was the first on in America.

CORRECTION: Humans are now able to modify our environments with technology. We have invented medical treatments, agricultural practices, and economic structures that significantly alter the challenges to reproduction and survival faced by modern humans. So, for example, because we can now treat diabetes with insulin, the gene versions that contribute to juvenile diabetes are no longer strongly selected against in developed countries. Some have argued that such technological advances mean that we've opted out of the evolutionary game and set ourselves beyond the reach of natural selection  essentially, that we've stopped evolving. However, this is not the case. Humans still face challenges to survival and reproduction, just not the same ones that we did 20,000 years ago. The direction, but not the fact of our evolution has changed. For example, modern humans living in densely populated areas face greater risks of epidemic diseases than did our hunter-gatherer ancestors (who did not come into close contact with so many people on a daily basis)  and this situation favors the spread of gene versions that protect against these diseases. Scientists have uncovered many such cases of recent human evolution. Explore these links to learn about:

The basic structural and functional unit of most living organisms.

He has done research on the biochemical analysis of rattlesnake venom using immunological techniques, snake ecology, and lizard ecology.

Professor of biopsychology at New Mexico State University in Las Cruces.